Preserving the Burgess Shale

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Science  16 Mar 2012:
Vol. 335, Issue 6074, pp. 1280
DOI: 10.1126/science.335.6074.1280-a
CREDIT: GAINES ET AL., PROC. NATL. ACAD. SCI. U.S.A. 109, 10.1073/PNAS.1111784109 (2012)

The Burgess Shale in British Columbia, Canada, is one of the most famous fossil fields in the world, containing remarkably preserved soft-bodied marine life from just after the Cambrian explosion. In most other fossil locations, these soft, fragile tissues usually degrade in sediments soon after they are deposited. By examining the composition of the overlying sediments and measuring the sulfur isotope composition of minerals surrounding the fossils from the Burgess Shale, the Chengjiang in China, and five other Burgess Shale–type deposits, Gaines et al. propose that the unique depositional environment and seawater chemistry composition were the primary factors that facilitated preservation. The seafloor at this time in Earth's history was probably anoxic and also deprived of other oxidants such as sulfate, which inhibited initial bacterial degradation of recently deposited organic remains on the seafloor. Soon after deposition, the fossils were entombed in fine-grained clay sediments and then capped by a thick carbonate cement—a result of increasingly alkaline seawater. The cement further prevented the exchange of oxidants into sediment pore water and allowed the organic matter to remain, eventually transforming into the carbonaceous compressions seen today.

Proc. Natl. Acad. Sci. U.S.A. 109, 10.1073/pnas.1111784109 (2012).

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